Method And Apparatus For The Production Of Gas From Air In Highly Flexible Gaseous And Liquid Form By Cryogenic Distillation

ABSTRACT

A method of producing at least one air gas using cryogenic distillation is provided. The expanded streams coming from the two turbines are combined and then split into two fractions. The first fraction is sent to the medium-pressure column of the system in gaseous form, whereas the second fraction is returned to the cold end of the heat exchange line. At a temperature T 4  below −100° C. and above T 2,  the second fraction is sent to a turbine where it expands up to a temperature T 5,  forming an air stream. This air stream is then warmed in the heat exchange line before being discharged into the atmosphere, so that the distillation is not disturbed. A liquid product is withdrawn from the column system as final product. The sole liquid product from the apparatus is liquid oxygen, but of course other products may be produced.

The conventional methods for producing air gases in liquid or gaseous form had distinct method architectures. Thus, there could be found:

-   -   an air separation apparatus that produced the main constituents         (O₂, N₂, Ar) at atmospheric pressure or slightly higher;     -   a step of compressing the products using compressors;     -   an independent nitrogen-liquefaction cycle that allowed all or         some of each of the constituents to be produced in liquid form         if necessary.

This configuration allowed a great deal of flexibility of use because each of the three “functions” implemented (separation, compression, liquefaction) could be performed or halted independently without affecting the operation of the other two.

Nonetheless, this configuration suffers from a significant lack of competitiveness, bearing in mind the very high cost of this design which requires one apparatus per function.

The most recent methods for producing air gases, which we term integrated methods, have the advantage that they can combine these three functions into a single equipment. So-called “pumped” apparatuses, including cycles of expanding air or possibly nitrogen, allow one and the same equipment to produce the constituents of air in pressurized gaseous form and in liquid form.

Among these, the methods involving staged vaporization in order to deliver products under pressure, as described in patent EP-A-0504029 or alternatively FR-A-2688052, are particularly attractive because they allow these functions to be combined from a single high-pressure air compressor. The energy efficiency of the whole is comparable with the traditional method and the investment is far lower.

By contrast, the flexibility of production is affected by the “three-in-one” combining of the functions and it becomes more difficult to operate or halt one function without affecting the whole.

It is an object of this invention to be able to combine the economic advantages of the integrated methods while at the same time retaining the flexibility offered by the traditional methods.

The invention provides a method of producing at least one air gas using cryogenic distillation in a system of columns comprising at least one medium-pressure column operating at a medium pressure and a low-pressure column operating at a low pressure, these being thermally coupled to one another and in which, in a first and a second operating mode:

a) all of a compressed air stream is raised to a high pressure, at least 5 bar above the pressure of the medium-pressure column, and purified at this high pressure, known as the main pressure;

b) this main pressure is possibly variable according to the products demanded;

c) a first part of the air stream at at least the main pressure is cooled in a heat exchange line down to an intermediate temperature thereof and is expanded in at least a first turbine;

d) possibly a second part of the air stream is expanded in at least a second turbine (21B) the admission and delivery conditions of which differ by at most 5 bar and by at most 15° C. or are identical in terms of pressure and temperature to those of the first turbine;

e) possibly the work provided by the first or a third turbine is used at least in part for the work required by a supercharger;

f) the admission pressure of the first turbine is very substantially higher than the medium pressure and possibly higher than the main pressure;

g) the delivery pressure of the first turbine is greater than or equal to the medium pressure, preferably substantially equal to the medium pressure;

h) a/the supercharger compresses at least a fraction of the air stream to a high pressure, greater than or equal to the main air pressure cooled in the heat exchange line down to a cryogenic temperature (≦100° C.), and returns the supercharged stream to the heat exchange line in which at least part becomes liquefied at the cold end and is then sent into the system of columns following expansion;

i) a pressurized liquid product from the system of columns is vaporized in the heat exchange line;

and in the first operating mode:

j) an auxiliary turbine admits a gaseous fraction of the air stream that has been cooled in the main heat exchange line;

k) the admission pressure of the auxiliary turbine is greater than or substantially equal to the main pressure, preferably at least 2 bar abs greater than or substantially equal to the main pressure;

l) the delivery pressure of the auxiliary turbine is greater than or substantially equal to atmospheric pressure, preferably substantially equal to the low pressure;

m) at least part of the air stream expanded in the auxiliary turbine is warmed in the heat exchange line;

n) some of the constituents of the air are produced in liquid form by way of end product;

and, in the second operating mode:

o) the air flow rate processed in the auxiliary turbine is reduced, by comparison with the stream processed in the auxiliary turbine in the first operating mode, possibly to zero, and

p) the production of liquid by way of end product is decreased by comparison with the production of liquid by way of end product in the first operating mode, possibly to zero.

According to other optional aspects:

-   -   all the turbines are braked by an air supercharger;     -   at least one supercharger coupled to one of the turbines admits         at ambient temperature;     -   of all the superchargers, only the supercharger mechanically         connected to the first turbine has an admission temperature of         below −100° C.;     -   the admission temperature of the first turbine differs by at         most 15° C. from the oxygen pseudo-vaporization temperature;     -   the flow rate of the incoming main air is reduced, during the         second mode, preferably by a flow rate at least equal to the         reduction during the second mode in the flow rate of air sent to         the auxiliary turbine;     -   the variation in main air flow rate is afforded by the variable         vanes of a compressor;     -   the variation in main air flow rate is afforded by starting         and/or stopping an auxiliary air compressor;     -   the main air pressure varies between the first mode and the         second mode;     -   the first part of the air is supercharged to a pressure higher         than the main pressure upstream of the first turbine so that it         enters the first turbine substantially at a pressure higher than         the main pressure;     -   the admission temperature of the auxiliary turbine is higher         than the admission temperature of the first turbine;     -   the air expanded in the auxiliary turbine is discharged to the         atmosphere.

Another aspect of the invention provides a unit for cooling and heating streams intended for and coming from a system of air separation columns comprising a heat exchange line, a first turbine, an auxiliary turbine, a supercharger, the heat exchange line comprising:

i) at least one passage for receiving a first purified air stream, the at least one passage for receiving a first purified air stream being connected to the supercharger,

ii) at least one passage connected to the delivery of the supercharger, the at least one passage connected to the supercharger being connected to the first turbine,

iii) at least two passages for receiving at least two fluids (35, 37) which become heated,

iv) at least one passage to receive a second purified air flow rate, the at least one passage for receiving the second purified air stream being connected to the admission of the auxiliary turbine and the delivery of the auxiliary turbine being connected to at least one passage for air to be heated.

The unit may be configured in such a way that, in operation, one of the following conditions is met:

i) the admission temperature of the auxiliary turbine is greater than the admission temperature of the first turbine

ii) the admission temperature of the auxiliary turbine is greater than the admission temperature of the supercharger

iii) the admission temperature of the supercharger is lower than the admission temperature of the first turbine

iv) the delivery temperature of the supercharger is greater than the admission temperature of the first turbine

v) the delivery temperature of the supercharger is greater than the delivery temperature of the auxiliary turbine.

What is proposed here is an improvement in the production flexibility of the single-machine type methods as described hereinabove:

-   -   either by offering the option of reducing or even canceling         liquid production of the units using a method like the one         described in EP-A-0504029;     -   or by offering the option of producing liquids efficiently using         methods such as those described in FR-A-2688052;     -   and by offering the option of doing one or the other reversibly,         and with good energy efficiency in both instances.

This method employs a known distillation system (medium-pressure and low-pressure columns thermally connected to one another, possibly an intermediate-pressure column and/or a mixing column and/or an argon mixture column, etc.) and involves at least two expansion turbines.

Two flow rates are at substantially equal pressure if their pressures differ only by the pressure drops.

The gaseous fraction of the air flow rate admitted by the auxiliary turbine is expanded beforehand in the first and/or the second turbine, possibly sent to the medium-pressure column and withdrawn from the medium-pressure column before being sent to the auxiliary turbine after having been warmed in the main heat exchange line.

In the first operating mode, the production of liquid product, all end products combined, constitutes 1% or 2% or 5% of the air stream sent to the columns (or to the column if only the medium-pressure column is supplied with air).

The invention will be described in greater detail with reference to the figures, which show air separation plants capable of operating on the method of the invention.

In FIG. 1, a compressed air stream 1 from a main compressor is supercharged in a supercharger 3 to a high pressure of at least 5 bar abs above the pressure of the medium-pressure column, this high pressure being known as the main pressure. This main pressure may, for example, be between 10 and 25 bar abs. At this main pressure, the stream 5 is then purified in respect of water and carbon dioxide (not illustrated). The total supercharged and purified air stream 5 is sent to a heat exchange line 7 where it is cooled down to a temperature T1. At that temperature, the stream 5 is split into two to form a stream 9 which becomes liquefied and is sent to the system of columns and a stream 11. The stream 11 leaves the heat exchange line 7 at the temperature T1 which differs by at most ±5° C. from the vaporization temperature of the pressurized oxygen 33 and is sent to a cold supercharger 13 to produce a stream 15 at a pressure very substantially higher than the medium pressure and possibly higher than the main pressure. The stream 15 at a temperature T2 as it leaves the cold supercharger is cooled in the heat exchange line 7 down to a temperature T3 higher than T1. At this temperature T3, the stream 15 is split into two streams 17, 19. The stream 17 is expanded in a turbine 21 from the temperature T3 close to the pseudo-vaporization temperature of the pressurized oxygen 33.

The admission pressure of the turbine 21 is equal to the delivery pressure of the supercharger 13 and therefore very substantially higher than the medium pressure (at least 5 bar higher) and possibly higher than the main pressure and the delivery pressure is greater than or equal to the medium pressure, preferably substantially equal to the medium pressure. The stream expanded to a pressure greater than or equal to the medium pressure, preferably substantially equal to the medium pressure, is sent to the system of columns by way of stream 25. The stream 19 continues to be cooled in the heat exchange line and is sent in gaseous form to the system of columns.

The cold supercharger 13 is driven by the turbine 21.

A residual nitrogen flow rate is warmed in the heat exchange line.

A stream of liquid oxygen 35, pressurized in a pump 33, becomes vaporized in the heat exchange line 7.

Optionally a liquid from the system of columns, other than the liquid oxygen, is pressurized, vaporized in the heat exchange line 7, and then used by way of pressurized product.

According to a first operating mode, an air fraction 25 is withdrawn from the purified air 5 at the main pressure and cooled in the heat exchange line 7. At a temperature T4 of below −100° C. and higher than T2, the fraction 25 is sent to a turbine 27 where it is expanded to a temperature T5 forming an air stream 29. This air stream is heated up in the heat exchange line.

A liquid product is withdrawn from the system of columns by way of end product 32. In the example, the only product of the apparatus is liquid oxygen but other products could obviously be produced in liquid form.

According to a second operating mode, the air flow rate 25 processed in the auxiliary turbine 27 is reduced possibly to zero, the flow rate of the incoming main air stream 1 is reduced by a flow rate at least equal to the reduction in the flow rate of the air sent to the auxiliary turbine 27 and the production of liquid 37 is decreased possibly to zero.

This variation in the flow rate of the stream 1 between the two operating modes is afforded by the variable vanes of a compressor and/or by starting and/or stopping an auxiliary air compressor.

These two operating modes may constitute the only operating modes of the apparatus or, alternatively, there may be other operating modes.

These may include a compression step (supercharger 3B) between the hot supercharging which raises the air to the main pressure and the cold supercharging, so that the cold supercharging is effected starting from a pressure above the main pressure.

As a preference, the turbine 21 is driven by the supercharger 13 and the supercharger 3 drives the auxiliary turbine 27. 

1-13. (canceled)
 14. A method of producing at least one air gas using cryogenic distillation in a system of columns comprising at least one medium-pressure column operating at a medium pressure and a low-pressure column operating at a low pressure, said at least one medium-pressure column and low-pressure column being thermally coupled to one another, comprising, in a first and a second operating mode: a) raising a compressed air stream to a high pressure, at least 5 bar above the pressure of the medium-pressure column, and purifying said air stream at this high pressure, b) varying said high pressure according to the demand for said at least one air gas; c) cooling a first part of the air stream in a heat exchange line to an intermediate temperature, and expanding said cooled first part in at least a first turbine; d) compressing at least a fraction of the air stream to a higher pressure in a supercharger, wherein said higher pressure is greater than or equal to the high pressure, cooling said higher pressure air stream n the heat exchange line down to a temperature less than or equal to −100° C., and returning the supercharged stream to the heat exchange line in which at least part becomes liquefied at the cold end and is then sent into the system of columns following expansion; e) vaporizing a pressurized liquid product from the system of columns in the heat exchange line; and in the first operating mode: f) admitting a gaseous fraction of the air stream into an auxiliary turbine, said fraction having been cooled in the main heat exchange line; g) expanding at least part of the air stream rate in the auxiliary turbine, warming said expanded stream in the heat exchange; and in the second operating mode: h) reducing the flow rate of the air stream processed in the auxiliary turbine, by comparison with the stream processed in the auxiliary turbine in the first mode; and i) decreasing the production of liquid by way of end product by comparison with the production of liquid by way of end product in the first mode.
 15. The method of claim 14, in which the turbine is braked by an air supercharger.
 16. The method of claim 14, in which the supercharger is coupled to one of the turbines, and admits at ambient temperature.
 17. The method of claim 14, the supercharger is mechanically coupled to the first turbine and has an admission temperature of below −100° C.
 18. The method of claim 14, in which the admission temperature of the first turbine differs by at most 15° C. from the oxygen pseudo-vaporization temperature.
 19. The method of claim 14, in which the flow rate of the incoming main air is reduced, during the second mode, preferably by a flow rate at least equal to the reduction in the flow rate of the air sent to the auxiliary turbine during the second mode.
 20. The method of claim 19, in which the variation in main air flow rate is afforded by the variable vanes of a compressor.
 21. The method of claim 19, in which the variation in main air flow rate is afforded by starting and/or stopping an auxiliary air compressor.
 22. The method of claim 14, in which the main air pressure varies between the first mode and the second mode.
 23. The method of claim 14, in which the first part of the air is supercharged to a pressure higher than the main pressure upstream of the first turbine so that it enters the first turbine substantially at a pressure higher than the main pressure.
 24. The method of claim 14, wherein a second part of the air stream is expanded in at least a second turbine the admission and delivery conditions of which differ by at most 5 bar and by at most 15° C. or are identical in terms of pressure and temperature to those of the first turbine.
 25. The method of claim 14, wherein the work provided by the first or a third turbine is used at least in part for the work required by the supercharger.
 26. The method of claim 14, wherein the admission pressure of the first turbine is substantially higher than the medium pressure.
 27. The method of claim 14, wherein the admission pressure of the first turbine is substantially equal to the medium pressure.
 28. The method of claim 14, wherein the delivery pressure of the auxiliary turbine is at least 2 bar greater or substantially equal to the high pressure.
 29. The method of claim 14, wherein the delivery pressure of the auxiliary turbine is substantially equal to the high pressure.
 30. The method of claim 14, wherein some of the constituents of the air are produced by way of end product in liquid form.
 31. The method of claim 14, further comprising reducing the flow rate of the air stream processed in the auxiliary turbine, by comparison with the stream processed in the auxiliary turbine in the first mode, to zero.
 32. The method of claim 14, further comprising decreasing the production of liquid by way of end product by comparison with the production of liquid by way of end product in the first mode, to zero.
 33. The method of claim 14, wherein the admission temperature of the auxiliary turbine is higher than the admission temperature of the first turbine.
 34. A unit for cooling and heating streams intended for and coming from a system of air separation columns comprising a heat exchange line, a first turbine, an auxiliary turbine, a supercharger, the heat exchange line comprising: i) at least one passage for receiving a first purified air stream, the at least one passage for receiving a first purified air stream being connected to the supercharger, ii) at least one passage connected to the delivery of the supercharger, the at least one passage connected to the supercharger being connected to the first turbine, iii) at least two passages for receiving at least two fluids (35, 37) which become heated, iv) at least one passage to receive a second purified air flow rate, the at least one passage for receiving the second purified air stream being connected to the admission of the auxiliary turbine and the delivery of the auxiliary turbine being connected to at least one passage for air to be heated.
 35. The unit of claim 34, configured in such a way that, in operation, one of the following conditions is met: i) the admission temperature of the auxiliary turbine is greater than the admission temperature of the first turbine; ii) the admission temperature of the auxiliary turbine is greater than the admission temperature of the supercharger; iii) the admission temperature of the supercharger is lower than the admission temperature of the first turbine; iv) the delivery temperature of the supercharger is greater than the admission temperature of the first turbine; v) the delivery temperature of the supercharger is greater than the delivery temperature of the auxiliary turbine. 